Versatile, rapid and accurate analytical techniques for the detection and quantification of water in a variety of materials remain an important and ubiquitous analytical problem. Indeed water is one of the most prevalent impurities in many industrial and consumer products and processes. In other cases, water is an essential component, the concentration of which must be known accurately and controlled.
The determination of water content in solvents and consumer products, including foods, pharmaceuticals, and industrial materials, is of great importance. Indeed analytical testing for the presence and concentration of water is one of the most frequent, important and ubiquitous measurements made in modern industrial society. Thus a versatile, simple and efficient analytical technique for the accurate quantification of water is imperative. The essentially universal presence of water requires accurate, facile and sensitive techniques to quantify it. While various techniques such as gravimetry, Karl Fisher titration (KFT), gas chromatography, near IR spectrophotometry, solvatochromic sensing, F-NMR spectroscopy, isotope ratio mass spectrometry (IRMS) and others have been reported in the literature, only a few methods are widely accepted and used.
Currently, the most commonly used method for water analysis is the KFT, which was first reported in 1935. In this titrametric method, I2 is reduced to HI in the presence of water. There are four components in the Karl Fischer reagent consisting of: iodine, sulfur dioxide, a suitable base (RN) (originally pyridine was used, but now imidazole is more common); and a suitable solvent such as methanol, ethanol, diethylene glycol monomethyl ether, etc.
The accepted mechanism of this two step reaction is:CH3OH+SO2+RN→[RNH]SO3CH3 H2O+[RNH]SO3CH3+I2+2RN→[RNH]SO4CH3+2[RNH]I
The end point is determined potentiometrically. Two types of KFT methods are used. They are the coulometric titration and the volumetric titration. Coulometric titration is used to detect trace amounts of water, ranging from 10 μg to 99 μg (1 ppm-5%), and it requires about 5 g or more of sample. Volumetric titration is used to detect water quantities higher than 1 mg (10 ppm-100%), and the amount of sample required varies from 0.1 mg to 500 mg. Therefore, prior knowledge of the approximate amount of water present in the sample is required in choosing the correct KFT method of analysis.
Although KFT is a well-established method, interference of side reactions, reagent instability, sample insolubility and pH issues prevent it from being accepted as a universal method. Variations on the basic KFT methodologies have been developed in an attempt to overcome these problems. However, many issues still remain, not the least of which is that the reagents degrade with time and there is residual water in all KFT reagents.
Another applied method for water detection is based on gas chromatography (GC). Early attempts using GC were mainly based on packed (molecular sieve) columns, involving both direct detection by thermal conductivity detector (TCD) and indirect detection (i.e. reacting water with calcium carbide to convert to acetylene) with a flame ionization detector (FID). Peak asymmetry, poor sensitivity, poor efficiency, strong adsorption of water and many solvents by the stationary phase, overlapping of the water peak by other larger peaks, and the inability to detect higher amounts of water limited its application in many cases. Attempts to eliminate peak asymmetry, using wide-diameter open tubular columns and capillary columns showed some improvement. Additionally, most conventional capillary column GC stationary phases are degraded by water.
One truly useful, broadly effective capillary GC method for water should meet several criteria including the following: 1) Water should not alter or degrade the stationary phase, thereby altering retention times and peak shapes; 2) There must be a considerable difference in the retention of water and most/all organic solvents especially when the solvent peak is very large relative to the water peak; 3) The water peaks should show good efficiency and symmetry; and 4) The water and solvent chromatogram should have sufficient separation space for an appropriate, baseline separated internal standard.
U.S. Pat. No. 8,182,581 to Armstrong et al reports diionic liquid salts comprising a dicationic or dianionic molecules and a counter-ion, and a method of using such diionic salts for separating one chemical from a mixture of chemicals. The methods comprise steps of providing a mixture of at least one first and at least one second chemical, and exposing that mixture to at least one solid support including a diionic liquid salt.
U.S. Pat. No. 8,097,721 to Armstrong et al describes triionic liquid slats comprising a tricationic or trianionic molecules and a counter-ion, and a method of using such triionic salts for separating one chemical from a mixture of chemicals. The methods comprise steps of providing a mixture of at least one first and at least one second chemical, and exposing that mixture to at least one solid support including a triionic liquid salt.